The case itself is important for a number of reasons, but is too lengthy for detailed analysis here. Highlights include:

First, the basic holding: the court remanded EPA’s primary annual standard for PM2.5, because EPA did not justify that the 15 ug/m3 standard was sufficient to protect public health with an adequate margin of safety. Second, the court also remanded EPA’s determination of the secondary, public welfare, standard for PM2.5.

The court gave great weight to the role of the Clean Air Science Advisory Committee (CASAC) and staff recommendations in the regulatory process. After this decision, EPA is going to think twice about choosing a regulatory course difference than that recommended by CASAC and staff. On balance, I think that this is a bad thing and more evidence of the collateral damage from the extreme positions taken by the Bush administration. After all, while the Clean Air Act sets some boundaries, these are ultimately policy decisions that should be made by the President and his or her chosen staff, not by a committee no one’s heard of or low-level staff.

Unlike the chaos created when the court vacated the CAIR regulations, the court appears to have learned its lesson. This time around, the court remanded the rule, but left the standard in place for now.

The court’s decision to remand the public welfare standard will have implications for current efforts to implement the its Regional Haze Rule. The extent to which this decision throws Haze Rule implementation back to the drawing board may not be known for some time.

How many more cases can the Bush administration lose after it’s already out of office? At least one. Greenwire reports today about speculation that this decision means that the EPA rules regarding the nitrogen oxide NAAQS may also be in trouble.

The interesting question in all this is the extent to which the abysmal record of the Bush EPA in defending its decisions in the courts will damage EPA’s credibility and thus result in a long-term weakening of the deference given EPA by the courts. At this point, my assumption is that, in the long run, these cases will be seen as an aberration and courts will resume their prior practice of granting EPA substantial deference. Of course, whether that is a good thing or not is a separate question.